1. Field of the Invention
The present invention is directed to integrated circuit inductors and transformers, and methods for making thereof, and more particularly, to spiral and toroidal inductors and transformers having high quality factor Q.
2. Discussion of the Prior Art
Many digital and analog components and circuits have been successfully implemented in silicon (Si) based integrated circuits (ICs). Such components include passive devices, such as resistors, capacitors, and inductors.
Implementing high quality factor (Q) inductors that operate at high radio frequencies (RFs) remain problematic in silicon based very large scale integration (VLSI) IC semiconductor chips. For microwave and wireless communications applications, it is desirable to integrate inductors and transformers monolithically on bulk silicon (Si), silicon-on-insulator (SOI), or silicon-on-sapphire (SOS) chips. For these applications, considerable innovation is necessary before adequate values of inductance (L), as well as high quality-factor (Q), are obtained. The quality-factor (Q) is given by equation (1): EQU Q=.omega..sub.0 L/R (1)
where, .omega..sub.0 =center or resonant angular frequency of oscillation;
L=inductance; and PA1 R=resistance. PA1 1. R. Naster et al., "Method for Fabricating Silicon-on-Sapphire Monolithic Microwave Integrated Circuits", U.S. Pat. No. 4,418,470 (1983); PA1 2. R. Stengel and C. Nejdl, "Planar Inductors", U.S. Pat. No. 4,494,100 (1985); PA1 3. R. Scranton and D. Thompson, "Capacitive Sensing Employing Thin Film Inductors", U.S. Pat. No. 4,648,087 (1987); PA1 4. J. Bhagat, "Miniature Inductor for Integrated Circuits and Devices", U.S. Pat. No. 5,070,317 (1991); PA1 5. N. Andoh et al., "Inductive Structures for Semiconductor Integrated Circuits", U.S. Pat. No. 5,095,357 (1992); PA1 6. A. Hubbard, "Integrated Circuit Inductor", U.S. Pat. No. 5,227,659 (1993); PA1 7. I. Saadat and M. Thomas, "Process for Making Microcomponents Integrated Circuits", U.S. Pat. No. 5,279,988 (1994); and PA1 8. C. Ahn et al., "A Fully Integrated Planar Toroidal Inductor with a Micromachined Nickel Iron Magnetic Bar", IEEE Trans. Compon. Packag. Manuf. Technol. A 17,463 (1994).
Most structures and methods used for fabricating high Q inductors and transformers in hybrid circuits, monolithic microwave integrated circuits (MMICs), or discrete applications (e.g., at larger dimensions, with better metals, on lossless substrates, etc.) are not readily compatible with silicon VLSI processing. The following references discuss conventional high Q inductor fabrication methods:
Conventional inductors and transformers on silicon, which have strictly planar structures and are fabricated with conventional silicon fabrication processes and material, suffer from several limitations. First, conventional aluminum interconnect technology has a relatively high metal resistivity and limited metal thickness, both of which lead to a relatively high resistance of the spiral coil. The high resistivity R lowers the quality factor Q, as seen from equation (1). Second, magnetic fields are strongly coupled to the Si substrate, which is a lossy conductor with large skin depth. Induced currents in the Si substrate act to oppose those in the spiral coil of the inductor and thus reduce the inductance L. Furthermore, energy is dissipated by the high resistivity of the Si substrate, which further decreases the quality-factor Q.
To date, Q values for inductors made by conventional methods are significantly below those achievable on a printed circuit board, or on gallium arsenide (GaAs) substrates with gold (Au) metalization. Another problem is that high inductance values (L), as are required in RF chokes for example, require a large silicon chip area. The large area requirement prevents miniaturization of chips. In addition, physically large RF chokes cannot operate at high frequencies, where the short wavelengths necessitates physically small and miniature components.
To overcome some of the limitations of spiral inductors, toroidal inductors are used. Conventional multilevel interconnect technology allows fabrication of solenoidal or toroidal inductor structures instead of an inductor having a spiral configuration. Toroidal inductors have the benefit of confining the magnetic flux, thereby minimizing substrate losses. However, conventional integrated toroidal inductors have comparably small inductance and Q values for a given silicon area, relative to the planar spiral inductor configuration. This is because the area enclosed by wire windings of conventional toroidal inductors is small due to the limit in vertical dimensions of the thin films used in conventional VLSI processing.
In light of the foregoing, there is a need for high Q inductors and transformers suitable for integration in IC chips, such as VLSI IC chips.